Electroluminescent multilayer optical information storage medium with integrated readout and composition of matter for use therein

ABSTRACT

An optical information storage medium has multiple information layers. On each layer, information is stored in the form of an electroluminescent material. The pits are organized into regions called pages. A specific page on a specific layer can be addressed through electrodes integrated into the medium. The medium can be read-only or WORM. For the WORM embodiment, a recordable electroluminescent material is combined with a quencher and a free radical generating compound that, when thermally degraded, generates free radicals to bleach the quencher. To amplify the light from an information layer, a photoconductive layer and an additional electroluminescent layer can be provided.

REFERENCE TO RELATED APPLICATION

This. application is a divisional of U.S. patent application Ser. No.09/502,769, filed Feb. 11, 2000, U.S. Pat. No. 6,819,649 which claimsthe benefit of U.S. Provisional Application No. 60/119,902, filedFebruary 12, 1999, whose disclosure is hereby incorporated by referencein its entirety into the present disclosure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical memory such as optical card or anoptical disk. In particular, this invention relates to anelectroluminescent device that in one embodiment has an organic orinorganic electroluminescent material capable of being driven by eithera positive or negative electric field, so that the device is capable ofbeing used with either a forward or a reverse electrical current. Mostparticularly, the invention relates to optical ROM and WORM devicesbased on an electroluminescent material device capable of being drivenby an alternating electric field. A material for a WORM medium is alsodisclosed.

2. Description of Related Art

Existing optical memory systems use two-dimensional data carriers withone or two information layers. Most of the previous technical solutionsto optical data recording propose registering changes in reflected laserradiation intensity in local regions (pits) of an information layer.Such changes can be a consequence of interference effects in reliefoptical discs of the CD-ROM type, burning of holes in the metal film,dye bleaching, local melting of polycarbonate in widely-used CD-Rsystems, change of reflection coefficient in phase-change systems, etc.

Three-dimensional, i.e., multilayer, optical storage systems providecomparatively higher storage capacity. However, they impose specificlimitations and requirements on the construction and features ofrecording media, the techniques for data recording and reading, andespecially the depth of the recording media.

Existing optical media, such as CD-ROM and DVD-ROM disks, are normallyread in reflection mode. However, for a multilayer storage medium to beread in reflection mode, every information layer must have an at leastpartially reflective coating. As a consequence, when a layer close tothe bottom of the medium is read, both the reading beam and thereflected beam pass through many such coatings, thereby attenuating thereflected beam to an unacceptable extent. Also, because existing opticalmedia are read with coherent radiation, both beams are subject todiffraction and interference distortions on pits and grooves of theinformation layers.

That is why multilayer fluorescent discs with fluorescent reading arepreferable, as they are free of partly reflective coatings. Diffractionand interference distortions are also reduced because of the incoherentnature of fluorescent radiation, its longer wavelength in comparison tothe reading laser wavelength, and the transparency and homogeneity(similar reflection coefficients of different layers) of the opticalmedia upon the incident laser and the fluorescent radiations. Thus,multilayer fluorescent discs have some advantages in comparison toreflective discs.

Optical memory cards are ROM or WORM media having a credit card form.The cards are highly durable, easily carried in a user's pocket, andunaffected by electrostatic and magnetic fields or heat. Optical memorycards surpass all other card technologies in terms of data capacity.

Information retrieval in fluorescent optical memory systems, inparticular, fluorescent ROM and WORM optical memory cards, is realizedwith the help of external light sources. Reading light is absorbed in afluorescent material held in information pits and excites thefluorescence of the material.

In an unrelated field of endeavor, inorganic electroluminescence deviceswere discovered by Destriau in 1936. Destriau observed that whensuitably prepared inorganic zinc sulfide phosphor powders activated withsmall additions of copper were suspended in an insulator and an intensealternating electric field (15 kV) was applied with capacitor-likeelectrodes, light emission resulted.

Electroluminescent research gained further impetus with the advent oftransparent conducting electrodes based on tin oxide (SnO₂) in the late1940's. Typically, early devices were composed of two sheets ofelectrically conductive material serving as electrodes, one a thinconducting backing and the other a transparent conductive film, placedon opposite sides of a plastic or ceramic sheet impregnated with theinorganic phosphor, such as zinc sulfide doped with small amounts ofcopper or magnesium. A transparent glass sheet placed next to thetransparent conductive sheet served as an outermost protectivesubstrate. Application of an alternating voltage to the electrodescaused an electric field to be applied to the phosphor. Each time thefield would change, radiation having a wavelength in the visible rangewas emitted.

Although a large amount of research and investigation was conducted onthe alternating current electroluminescent devices, the devices neverachieved practice application although they were originally highlytouted as a room lighting sources. Unfortunately, at high brightnesslevels the AC electroluminescent devices exhibited a very short life,and after about 1963, most of the research into the ACelectro-luminescence devices was severely curtailed.

The most recent efforts in this area have been directed to a molecularcarbon (e.g., a form of carbon known as fullerene-60) system. The highvoltage drive requirements, the associated high cost of drive circuitry,poor stability and lack of color capability have made these devices costprohibitive.

Two other inorganic devices, 1) direct current (DC) inorganicsemiconductor light emitting diodes (LEDs) and 2) fluorescent ion dopedinorganic semiconductor thin film devices, trace their origins to themid-fifties. Light emitting diodes based on forward biased inorganicsemiconductor p-n junctions are limited to small area applications as aresult of color, efficiency and cost limitations. The other inorganicdevices, fluorescent ion-doped inorganic semiconductor thin filmdevices, require high operating voltages to accelerate electrons orholes to sufficient energies in order to excite or ionize thefluorescent ion centers. Such high operating voltages result inthin-film instability and failure of such devices.

Electroluminescent organic materials include both molecular and polymerforms These materials include light emitting polymeric polypyridinessuch a poly(p-pyridines), co-polymers such as poly(phenylenevinylenepyridylvinylene) and molecular light emitters such as8-hyrooxyquinoline. Insulating materials include a wide variety ofceramics such as aluminum oxide and inorganic and organic materials suchas polysilane, polymethylmethacyline, nylon, cimeraldine base (aninsulating polyaniline polymer) and organic molecules such as2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4 oxdiazole. Electrodes can befashioned from any suitable conductor including, but not limited to, awide variety of conducting materials including 1) indium tin oxide, 2)metals such as gold, aluminum, calcium, silver, copper, and indium, 3)alloys such as magnesium-silver, 4) conducting fibers such as carbonfibers, and 5) conducting organic polymers such as conducting dopedpolyaniline and conducting doped polymole.

In typical applications where the device is used for lighting anddisplay, at least one of the electrodes is fashioned from a transparentmaterial such as indium tin oxide or a partially transparent materialsuch as conducting doped polyaniline. The insulator between the lightemitting material and the transparent or partially transparent electrodeis also transparent or partially transparent and fabricated from anoptically clear insulating polymer such as polymethylmethacrylate or apartially transparent insulating polymer such as the insulatingemeraldine base form of polyaniline. Partially transparent electrodesand insulators can be used to advantage to filter or clip unwantedportions (frequencies) of light emitted from the organic light emitter.

For ease of manufacture and insulation purposes, it is preferable toform the device on a substrate which also serves to protect andtypically insulate the device during use. Glass and clear insulatingplastic are preferable when the device is used for lighting and displaypurposes. The AC driven symmetrical device is especially suitable forlight emissions from both sides of the device in which case bothinsulators and electrodes are at least partially transparent as well asany insulating substrates that may be used with one or both electrodes.

In the last decade, there has been an emerging interest in directcurrent (DC) molecular and polymer electroluminescence devices. Avariety of organic molecules and conjugated polymers, copolymers andmixtures have been found to exhibit electroluminescent properties.Light-emitting diodes incorporating these materials have demonstratedall of the necessary colors (red, green, and blue) needed for displayapplications. However, a need continues to exist to lower the deviceoperating voltages and to increase their light-emitting (output)efficiency. Further improvements in charge injection and the balancingof charge transport are needed. Because of the asymmetry of the deviceconfiguration, efficient charge injection occurs only in one direction(forward DC bias). Under reverse bias, most of the devices eitherdegrade quickly or show very poor performance. Although efforts havebeen made to improve the charge injection efficiency by the use of lowwork function electrodes such as calcium or magnesium and the use of anelectron transporting material to improve negative charge (electron)injection, such devices continue to be operational in only onedirection. Similarly, efforts have been made to improve charge injectionefficiency by the use of high work function electrodes such as indiumtin oxide (ITO) or gold and the use of hole transporting materials toimprove positive charge (hole) injection. Such devices also continue tobe operational in only one direction.

SUMMARY OF THE INVENTION

A need continues to exist to develop bipolar devices that have low turnon and operating voltages, flexibility, large area, high operating(light output) efficiency and low production cost. It is therefore aprimary object of the invention to provide such devices.

It is another object of the present invention to store information influorescent multilayer cards, tapes and any other optical media whichhave no moving parts (such as ROM cards and WORM cards).

It is still another object of the present invention to provide a 3-Doptical memory device for recording and reading information.

It is yet another object of the present invention to provide a 3-Doptical WORM (write once, read many) memory device for information inwhich writing is implemented by photochemical decomposition of aquencher of electroluminescence and reading is implemented byregistration of electroluminescence, as well as to provide a compositionof matter for use in such a WORM drive.

To achieve the above and other objects, the present invention isdirected to a multilayered optical memory card whose information pitsare filled with electroluminescent materials. Information readout isrealized by applying voltage to a desired information page which causesfluorescence irradiation. Such an optical card allows the use ofintegrated electronics.

Data on each layer are organized into a plurality of regions or patches(called pages). A data page is selectively illuminated and imaged ontoan array of light sensors, e.g., by an array of lenslets. Transmittedpage data, in this case light passed through the transparent bitlocations on the data layer, strike different ones of the light sensors,thereby outputting a pattern of binary bits in the form of electricaldata signals.

The data storage and retrieval system of the present invention isembodied in ROM devices and WORM devices as described more fully belowin the detailed description. A preferred embodiment of the inventioninvolves forming the data layer as a bonded structural unit or card,much like a sandwich of different layers of material, to thereby fix theoptical distances and registrations of these elements. This bonded datacard structure minimizes the adverse optical effects of differentialthermal expansion between the data layers and allows for an exceedinglydense data pattern.

There is a great need in optical devices which will have not movingparts. The main advantage of the this design is the increased readingspeed. Another advantage is improved reliability of the drive forreading the optical medium.

The present invention offers the following advantages:

-   -   Absence of interlayer and interpage crosstalk (as        electroluminescence is excited only in the certain page);    -   Ultrahigh data density (1 bit/mcm³ or 100 Gbytes/cm³);    -   Possibility of color electroluminescent card that means higher        information capacity; and    -   High information bit rate (more than 1 Gbit/sec).

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be set forth indetail with reference to the drawings, in which:

FIG. 1 shows a plan view of the layers of an optical card according tothe first and second embodiments of the invention;

FIG. 2 shows a cross-sectional view of the layers of an optical cardaccording to the third embodiment of the invention;

FIG. 3 shows a cross-sectional view of the layers of an optical cardaccording to the fourth embodiment of the invention;

FIG. 4 shows the electroluminescent spectrum of poly(p-phenylenevinylene) showing wavelength in manometers on the horizontal axis andelectroluminescent intensity in arbitrary units on the vertical axis;

FIG. 5 shows a schematic of one page of information in any of thepreceding embodiments and the manner in which it is addressed;

FIG. 6 shows a modification of FIG. 5 that increases informationcapacity by the use of multiple colors; and

FIG. 7 shows a cross-sectional view of the layers of an optical cardaccording to a fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will now be set forth in detail with reference tothe drawings, in which like reference numerals refer to like componentsthroughout.

FIG. 1 generally shows a first embodiment for an optical ROM or WORMdevice with electroluminescent reading of information. The device, shownas a card 100, includes an information layer 101 with thickness 0.5-40microns made of a suitable plastic material, preferably polycarbonate,polychlorovinyl, or polymethylmethacrylate, which serves as a mechanicalbase for the card 100. Every information layer includes large numbers ofdata pits 104 lying in the same place. Every pit contains a layer ofelectroluminescent material, in this example poly(p-phenylene vinylene)on the bottom of the pit dissolved in a plastifier.

The electroluminescent material is a polymer selected from the group ofelectroluminescent polymers consisting of polypyridines,polypyridyvinylenes, polyphenylenes, polyphenylenevinylenes,polythiophenes, polyvinylcarbazoles, polyfluorenes,polynaphthalenevinylenes, polyphenyleneacetylenes,polyphenylenediacetylenes, polycyanoterephthalylidenes,polyphenylenebenzobisthiazoles, polybenzimidazobenzophenanthrolines,polypyridine copolymers, polypyridylvinylene copolymers, polyphenylenecopolymers, polyphenylenevinylene copolymers, polythiophene copolymers,polyvinylcarbazole copolymers, polyfluorene copolymers,polynaphthalenevinylene copolymers, polyphenyleneacetylene copolymers,polyphenylenediacetylene copolymers, polycyanotherephthalylidenecopolymers, polyphenylenebenzobisthiazole copolymers,polybenzimidazobenzophenanthroline copolymers, and mixtures thereof.

Voltage is applied using a wire netting of transparent electrodes. Eachanode 102 is a thin layer of indium tin oxide (ITO), and each cathode103 can be any conductor providing electron injection into the adjacentinformation layer 101. The voltage source can be a direct voltage sourceconnected in either direction, that is, connected to produce a currentthat flows in either a forward or a reverse direction. Preferably thevoltage source is an alternating voltage source producing an alternatingcurrent. A typical square surrounded by the wire netting is about 1mm².The pixellation and patterning of the emission is achieved through thedistribution of the pits 104 and not through patterning of theelectronics. The operation voltage is about 5V, and the current densityis in the range of 1-500 mA/cm². Emission from the layer(electroluminescence) is detected by a CCD matrix via a lens. While oneset of layers 101, 102, 103 is shown, it will be understood thatmultiple such sets of layers can be stacked on one another, in whichcase they can be separated by insulating layers. Also, a substrate and atop protective layer can be provided, as known in the art. Thesymmetrical arrangement of electrodes allows the use of alternatingcurrent.

The second embodiment for optical device for ROM or WORM withelectroluminescent reading of information has the same basic arrangementof layers as shown in FIG. 1; therefore, the description thereof willnot be repeated. Such a device includes an information layer withthickness 2-20 microns made of a sol-gel film which serves as amechanical base for the medium. Every information layer has many datapits lying in the same plane. Every pit contains a inorganic phosphor,e.g., ZrS as an electroluminophore.

Voltage is applied using the wire netting of transparent electrodes. Awide variety of electrode materials can be used to the electrodes of thepresent device including 1) indium tin oxide, 2) metals, includingcalcium, aluminum, iodine, gold, copper, and silver, 3) alloys, such asmagnesium silver, 4) conducting non-metals such as carbon and graphitefibers and filaments, and 5) conducting polymers such as polyamine dopedwith dopants such as camphor sulfonic acid, hydrochloric acid, tosylicacid, and docecylbenzene sulfonic acid and polypyrrhole doped withphosphorous hexafloride and p-toluenesulfonate. As used here aconducting polymer is a polymer with a conductivity of more than about10⁻²S/cm and preferably more than about 10⁻¹ S/cm.

For many applications, it is desirable to use a transparent electrodesuch as indium tin-oxide and a transparent insulator such aspolymethylmethacrylate. It is often preferable to fabricate the deviceusing a suitable substrate. Typically, the substrate is an insulatingmaterial such as a plastic or glass. Glass and optical plastics such apolymethylmethacrylates, polycarbonates, and polystyrene may be usedwhen light transmission is desired. Inorganic materials permit the useof alternating voltage. A typical square surrounded with the wirenetting is about 1 mm². The operation voltage is about 45V, and thefrequency is 400 Hz. Emission from the information layer(electroluminescence) is detected by a CCD matrix via a lens. Themodifications proposed above for the first embodiment can also beapplied to the second embodiment.

FIG. 2 shows a third embodiment for a ROM or WORM optical device withelectroluminescent reading of information. The embodiment includes amultilayer structure 200 in which every data layer has anelectroluminescent layer 201 functioning as an information layer and apair of transparent electrodes 202 and 203 with the electroluminescentlayer 201 therebetween.

The electroluminescent layer 201 is made of a suitable plastic material,preferably polycarbonate, polychlorvinyl, or polymethylmethacrylate,which serves as a mechanical base for the medium. Everyelectroluminescent layer 201 has many data pits 204 lying in the sameplane. Every pit 204 contains a layer of electroluminescent materialsuch as poly(p-phenylene vinylene) on the bottom of the pit dissolved inplastifier.

The anode 202 is a thin layer of transparent ITO, and the cathode 203 ismade of Al. Each of the electrode layers 202 and 203 is an array ofconductors with X and Y orientations, thereby allowing selectiveaddressing of a portion of the electroluminescent layer 201. The sameaddressing technique can be used in the first and second embodiments.The operation voltage is about 5V, and the-current density is in therange of 1-500 mA/cm². Emission from the layer electroluminescence isdirected onto a CCD matrix by a lens.

The fourth embodiment for a WORM optical device has a multilayerstructure 300 in every information layer 301 has one or more spiralgrooves 302 (or grooves of another shape, or arrays of pits) lying inthe same plane. Every groove 302 contains a thin (10-100 nm) activelayer 303 of a recording medium including an electroluminescentmaterial, poly(p-phenylene vinylene), and a dye composition. Informationrecording in such a device is done by a focused laser beam which scansthe surface of the active layer. The laser radiation is absorbed by adye that transforms the energy of the laser into heat, causing physicaland chemical changes in the active layer. The constituents of the activelayer are:

1. Electroluminescent material—poly(p-phenylene vinylene).

2. Compound capable of generating free radicals when thermally degraded(azodiizobutyronitryle). These free radicals react with the quencher ofthe electroluminescent material and cause its bleaching.

3. Fluorescent quencher for electroluminescent material. This compoundis selected from the group of dyes consisting of 3,3′-diethylexadicarbocyanine iodide,4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4-pyran, and1,1′-diethyl-2,2′-carbocyanine iodide.

Other layers 304 can separate the information layers 301. Such otherlayers can include electrodes, insulators, or the like.

Information recording in the device is implemented by photobleaching thequencher enabling the increasing the initial fluorescent signal. Inaddition, the active layer composition can contain compounds whichpromote or suppress photothermic bleaching, plastics, and organicreducers which prevent free radicals deactivation by oxygen.

The typical elecroluminescent spectrum of poly(p-phenylene vinylene) invinyl acetate on a PMMA substrate in the presence of 0.004M ofazodiizobutyronitryle and 0.035M of 3,3′-diethyloxadicarbocyanine iodide(absorber and quencher) is shown in FIG. 4 (wavelenth in nanometers onthe horizontal axis and electroluminescence intensity in arbitrary unitson the vertical axis). The spectrum was measured using an HR340spectrograph.

In FIG. 5, the schematic of one of information pages of a many-layeredelectroluminescing optical card according to any of the precedingembodiments is shown. A substrate 501 supports a transparent anode 502,a light emitting layer 503, and a cathode 504 in that order. The lightemitting layer 503 can be separated from the anode 502 and the cathode504 by a hole transport layer and an electron transport layerrespectively; for the sake of clarity, neither of those layers is shown.A voltage V applied between the anode 502 and the cathode 504 causes thelight emitting layer to emit light L, which is focused by an objectivelens 505 onto a CCD array or other detector 506, so that information isreproduced. Any page of information on any layer can be addressed byapplying the voltage V to the specific anode 502 and cathode 504associated with that page.

The information capacity of an electroluminescent 3D optical memory canbe increased with the help of electroluminescing layers with differentelectroluminescence wavelengths. As shown in FIG. 6, a stack 600 oflayers includes multiple layers 601, of which the lowest layer 601 restson a substrate 602. The layer 601 includes layers 603, 604, and 605 thatundergo electroluminescence at different wavelengths in response toaddressing voltages applied through the appropriate electrodes 607. Anintermediate layer 606 separates the layers 601 from one another.

Energy characteristics and the information retrieval rate of a 1 GBelectroluminescent optical card as a function of the CCD accumulationtime are set forth in the table below. The pit square is 1 μm², thefluorescence wavelength is 555 nm, the pit brightness is 3000 cd/M², andthe objective numerical aperture is 0.45.

Accumulation time of CCD Total number of photons camera, ms incident oneach pixel Data rate, MB/s 1000 10⁷ 0.25 100 10⁶ 2.5 10 10⁵ 25 1 10⁴ 250

A fifth embodiment will now be set forth. As shown in FIG. 7, the fifthembodiment is directed to a card 700 having information layers 701, eachwith its electrode layers 702, separated by insulating layers 703. Theconstruction and operation of such layers can be as in any of thepreceding embodiments. In addition, a photoconductive layer 704 and anelectroluminescnet layer 705 are provided above all of the informationlayers 702.

A single information layer 702 has a relatively small thickness,typically less than 0.1 micron, and the applied voltage can be reducedfrom that used in the previous embodiments.

The photoconductive layer 704 and the electroluminescent layer 705together make up an imaging layer with two optically transparentelectrodes 706. The layers 704 and 705, and thus the imaging layer, aredivided into pixels, each of size comparable with the size of a pixel inthe information layer.

A voltage is applied to the imaging layer through the electrodes 706.The voltage chosen is not by itself sufficient to causeelectroluminescence from the electroluminescent layer 705. When a pagefrom an information layer 702 is activated, its weak light induces aphotocurrent in the photoconductive layer 704, thus leading to aredistribution of the voltage between the photoconductive andelectroluminescent layers 704 and 705. Most of the voltage is nowapplied to the electroluminescent layer 705, thereby causingelectroluminescence in the electroluminescent layer 705. Theelectroluminescence is induced only in the pixels of the imaging layersituated above the pits of the information layer from the radiatingpage. Since the distance between information layers and theelectroluminescent layer 705 is small compared to the wavelength ofelectroluminescent light, the cross-talk between different pits issmall. As a result, the data stored in the information layer arefaithfully transferred to the imaging layer. The suggested method ofreadout of the weak information signal by using the imaging layer servesas an effective “optical transistor.”

The cards and other media according to the embodiments disclosed abovehave the following advantages:

-   -   Absence of interlayer and interpage crosstalk (as        electroluminescence is excited only in a desired page);    -   Ultrahigh data density (1 bit/mcm³ or 10(Gbites/cm³);    -   The possibility of a color electroluminescent card, providing        higher information capacity; and    -   High information bit rate (more than 1 Gbit/sec).

While various preferred embodiments have been set forth in detail above,those skilled in the art who have reviewed the present disclosure willreadily appreciate that other embodiments can be realized within thescope of the invention. For example, any suitable materials can be usedto form the various parts of the card, and any suitable arrangement ofelectrodes can be used. Therefore, the present invention should beconstrued as limited only by the appended claims.

1. A write once, read many information storage medium comprising: aplurality of information layers, each of the plurality of layers havingat least one groove; and in each groove of each of the plurality ofinformation layers, an active layer comprising: an electroluminescentmaterial; a fluorescent quencher for the electroluminescent material;and a free radical generating compound that, when thermally degraded,generates free radicals that react with the quencher to bleach thequencher.
 2. The write once, read many information storage medium ofclaim 1, wherein each groove is spiral.